**4. Conclusions**

Exhausted olive pomace is an interesting source of natural antioxidant compounds, which can be extracted as a first step in a biorefinery strategy for valorisation of this agro-industrial residue. This work investigated the impact of ultrasound assisted extraction conditions on TPC, TFC, and antioxidant activity of EOP while using RSM. The optimal conditions for UAE were determined at 43.2% ethanol concentration, 70% ultrasound amplitude, and 15 min extraction time, yielding an optimal extract with higher phenolic and flavonoid content, as well as higher antioxidant activity than those previously determined for other by-products of the olive oil industries, such olive oil or olive tree pruning also extracted by ultrasound.

**Author Contributions:** I.R., E.R. and E.C. designed the experiments and revised the manuscript; J.C.M.-P. and I.G.-C. performed the experimental part and wrote the draft version of the manuscript; M.B., B.G. and I.R. supervised the experimental work and analysed and discussed the results. All authors approved the final version.

**Funding:** This research was funded by the Spanish Ministerio de Economía y Competitividad- Agencia Estatal de Investigación and Fondo Europeo de Desarrollo Regional. Project Reference ENE2017-85819-C2-1-R.

**Acknowledgments:** José Carlos Martínez Patiño gratefully acknowledges financial support provided by the Universidad de Jaén Doctoral School. Irene Gómez-Cruz expresses her gratitude also to the Universidad de Jaén for financial support (grant R5/04/2017).

**Conflicts of Interest:** The authors declare no conflict of interest.
